This invention relates to a process for producing p-cresol by direct oxidation of p-tolualdehyde with a peroxide such as hydrogen peroxide or performic acid.
p-Cresol is an important raw material for plastic and also important intermediate for industrial chemicals, medicines, agricultural chemicals, etc.
p-Cresol can be produced according to a separation process from tar acids, or various synthesis processes including the cymene process.
Generally, alkylphenols are not so different from one another in physical properties, for example, between their isomers, or even between their homologs. p-Cresol of high purity is thus hard to obtain according to the separation process.
On the other hand, the synthetic processes also have disadvantages. For example, isomers are formed as by-products or the wastes must be disposed in the sulfonation-alkali fusion process or complicated apparatuses are required in the cymene process.
Apart from the processes described above, a process based on the Baeyer-Villiger reaction is well known for synthesizing cresyl formate by oxidizing p-tolualdehyde with a peracid such as hydrogen peroxide. The said process is suitable for synthesizing p-cresol of high purity, and is regarded as an attractive process for preparing p-cresol in view of the recent success in commercial production of p-tolualdehyde by carbonylation of toluene. In this connection, a process for oxidizing p-tolualdehyde with a carboxylic peracid having a pKa below 4 in water has been proposed (see U.S. Pat. No. 3,927,123), wherein, when performic acid derived from hydrogen peroxide is used, the reaction proceeds as follows: ##STR1## Overall reaction equation can be expressed as follows: ##STR2## In the process, cresyl ester and water are formed by oxidizing p-tolualdehyde, and a step for hydrolyzing the ester is indispensable for the preparation of p-cresol.
Furthermore, the water content of the medium increases as the reaction proceeds, resulting in lowering in the reaction rate. Due to these disadvantages the process is still unsatisfactory as a commercial process in spite of its distinguished selectivity.
As a result of extensive studies made of a commercial process for preparing p-cresol from p-tolualdehyde, the present inventors have found that p-tolualdehyde can be directly converted to p-cresol by oxidizing p-tolualdehyde in formic acid solvent under specific conditions, and have established the present invention.